The present disclosure relates generally to a separation pad and media separator used for handling sheets of media. More particularly, the present disclosure relates to a separation pad that includes first and second friction regions, and a media separator that supports the separation pad for movement relative to the media separator in a sheet feeding direction through a nip formed by the media separator. The first and second friction regions selectively engage a sheet of media fed through the nip, with a retard/separation force determined by the bias force, to retard and control a feeding operation of the sheet of media fed through the nip, and to feed plural sheets of media through the nip one sheet at a time. The present disclosure further relates to a media separator mechanism including a media pick and the separation pad and media separator.
A separation pad, media separator and media separator mechanism of the present disclosure have particular utility in a media handling system that handles a plurality of types of media. However, the separation pad, media separator and media separator mechanism of the present disclosure may have utility in any apparatus that handles sheets of media.
Media handling systems are known. Examples include readers, scanners, printers, copiers, facsimile machines and the like. Such media handling systems handle a variety of media having a variety of different physical characteristics. Examples of paper media include lightweight stock, standard stock, bond, cardstock, glossy, envelopes and the like. Examples of other media include transparencies, films, labels and the like. These media have various physical characteristics or properties, including strength, thickness, surface coefficients of friction and the like, that can vary over a wide range. System designers must design media handling systems to accommodate these variations in physical characteristics.
Media separators are known. Generally, a media separator cooperates with a media pick to form a nip in a feed path of a media handling apparatus to control a feed operation of a sheet of media through the nip. For example, a media separator and media pick may form a media pick and separation mechanism, for picking up and feeding a plurality of sheets of media from a media stack on a media tray, one sheet at a time. As used herein, a media pick generally is a device that frictionally engages a top surface of a sheet of media and provides a frictional force for driving the sheet of media into and through a nip in a feed path. As used herein, a media separator generally is a structure or device that frictionally engages a bottom surface of a sheet of media fed through the nip. During a feeding operation, the media separator applies a retard/separation force to a sheet of media in contact with the media pick sufficient to control the feeding operation of the sheet of media through the nip; the media separator applies a retard/separation force to a sheet of media other than a sheet of media in contact with the media pick sufficient to separate plural sheets of media simultaneously fed into the nip, to feed the plural sheets of media one at a time.
Conventional media separators generally come in one of two forms. In one form, the media separator includes a fixed contact surface including a friction surface or separation pad that opposes the media pick. The contact surface frictionally engages each sheet of media in the nip to retard and control a feeding operation of the sheet of media fed through the nip. In a second form, the media separator includes a retard roller having a rotation surface or tire that opposes the media pick. The retard roller rotates through the nip against a reverse-bias torque to retard and control a feeding operation of the sheet of media fed through the nip. The retard roller can be undriven (passive) or driven in a reverse direction relative to the media pick (active).
Design criteria of a simplified media pick and separation system are described here by way of example. To advance a top sheet of media through a nip, the media pick must generate a drive force Fdrive greater than the retard/separation force Fret/sep of the media separator. To prevent simultaneous feeding of multiple sheets through the nip, the media separator must generate a retard/separation force Fret/sep on a bottom sheet of media greater than the potential friction force between the individual sheets of media Fsheet-sheet. Thus, the following relationship must be satisfied:Fdrive>Fret/sep>Fsheet-sheet  (1)
The drive force Fdrive depends directly on the nip force Fnip and the coefficient of friction of the media pick on the sheet of media μpick-media, as follows:Fdrive=Fnip×μpick-media  (2)
Materials suitable for use as a media pick limit the available drive force. These materials typically include ethylene propylene diene monomer (EPDM), urethane, latex and like elastomers. Common values for the coefficient of friction of media picks are around 2.0. However, contamination and wear can lower this value to 1.5 or less. In this regard, values for coefficients of friction (μ) used in this application refer to values determined according to the American Society of Testing and Materials (ASTM) standard methods. Those skilled in the art will recognize that coefficients of friction may vary depending on the conditions and method of detection.
The sheet-to-sheet frictional force Fsheet-sheet depends on the nip force Fnip and the coefficient of friction between the sheets of media μsheet-sheet, as follows:Fsheet-sheet=Fnip×μsheet-sheet  (3)
A system designer has substantially no control over the sheet-to-sheet frictional force. The system user selects the media for each application. The coefficient of friction for standard office media is about 0.5. However, media coatings, static charge buildup, and other factors can effectively raise this value to 1.0 or higher.
A system designer must design the media separator to generate a retard/separation force that fits within the window between these two limits—the drive force and the sheet-to-sheet frictional force—to reliably separate plural sheets of media simultaneously fed into the nip. If the retard/separation force is too close to the frictional drive force, then media pick errors/failures will occur. If the retard/separation force is too close to the sheet-to-sheet friction force, then multiple sheet pick errors will occur. Also, the optimal relationship of drive force to retard/separation force is different for each media, and often the overlap between acceptable settings is small.
A separation pad is an inexpensive and compact media separator. Conventional separation pads-generally use a stationary friction surface to form a nip with a media pick. In such a mechanism, the retard/separation force Fret/sep is directly related to the nip force Fnip and the coefficient of friction of the separation pad with the media μpad-media, as follows:Fret/sep=Fnip×μpad-media  (4)In a separation pad mechanism, the nip force thus directly affects each of the drive force, the retard/separation force and the sheet-to-sheet force.
Accordingly, although a separation pad mechanism has utility in many applications, it has a drawback in that the only independent variable affecting the separation force that a system designer can manipulate is the coefficient of friction of the separation pad. That is, this mechanism provides a narrow window of acceptable coefficients of friction. A system designer may have difficulty finding a material for the separator pad that meets the system design criteria. In addition, system wear and contamination can change the coefficient of friction of a material over time, causing a decrease in system performance or system failure.
A retard roller is a more reliable media separator. A retard roller generally is a roller that cooperates with the media pick to form the nip, and resists turning relative to the media pick/sheet of media by some known amount of torque Tretard. This mechanism thus provides a designer with an additional variable to adjust the retard/separation force. Specifically, the retard/separation force Fret/sep in this mechanism is the lesser of:Fret/sep=Tretard/rroller  (5)andFret/sepFnip×μroller-media  (6)where rroller is the radius of the retard roller, and where μroller-media is the coefficient of friction between the retard roller and the sheet of media.
A system designer thus may choose to use a retard roller material having a coefficient of friction sufficiently high to make the first equation applicable. This makes the retard/separation force Fret/sep independent of the nip force, which permits the system designer to independently manipulate the media pick drive force and retard/separation force.
Although retard roller mechanisms have utility in many applications, they have a drawback in that they require additional elements, such as drive motors, controllers, clutch mechanisms and the like, which require additional space, technical maintenance and cost.
Various media separator mechanisms using separation pads and retard rollers are known. The following three examples illustrate media separator mechanisms using various media picks and separation pads or retard rollers.
U.S. Pat. No. 3,768,803 discloses a sheet feeder including a media pick and separation pad for separating sheets of media to be fed one at a time through a nip formed between the media pick and the separation pad. The media pick includes an endless sheet separation belt driven around plural rollers. One roller, the pick roller, is provided adjacent an edge of a stack of sheets of media (media stack) so that the sheet separation belt is in press contact with a top surface of the top sheet of media in the media stack at a region of edge contact. The separation pad includes a jaw and tongue member that opposes the pick roller and sheet separation belt to form a mouth of the nip, and a frictional surface that opposes the sheet separation belt in a region between the pick roller and another roller to form a queuing throat of the nip.
The sheets of media are fed by frictional driving force. The separation belt by frictional force pulls the top sheet of media into the mouth of the nip; the top sheet engages the jaw and tongue member of the separator pad and is guided into the throat of the nip, where the top sheet of media engages the frictional surface of the separator pad with a frictional force that retards movement of the top sheet through the throat of the nip. The top sheet of media by frictional force in turn pulls the next adjacent sheet of media (second sheet) into the mouth of the nip; the second sheet engages the jaw and tongue member and is guided into the throat of the nip, where the second sheet engages the frictional surface of the separator pad with a frictional force that retards movement of the second sheet into the throat of the nip. Each sheet of media exerts a similar (although gradually smaller) frictional force and pull on a successive sheet of media in the media stack. In this manner, the separation belt pulls plural sheets of media into the mouth and queuing throat of the nip, and into frictional engagement with the frictional surface of the separator pad, and the plural sheets of media in the queuing throat engage the frictional surface of the separation pad in a stepped or staggered manner.
A desired one-at-a-time sheet feeding operation is obtained by selecting materials having suitable coefficients of friction and selecting suitable contact pressure forces. The frictional (driving) force between the separation belt and the top sheet of media is determined by the coefficient of friction of the separation belt, the coefficient of friction of the sheet of media, and the contact pressure between the media pick roller/separation belt and the media stack. The frictional (driving) force between adjacent sheets of media is determined by the coefficient of friction of each sheet of media and the contact pressure of the media pick roller/separation belt on the media stack. The retard force for each sheet is determined by (1) a frictional force between each sheet and its successive sheet of media in the stack, which is determined by the coefficient of friction of the sheets of media and the contact pressure of the media pick roller on the media stack, and (2) the frictional (retard) force between the frictional surface of the separation pad and each sheet of media in the queuing throat of the nip, which is determined by the coefficient of friction of the frictional surface of the separation pad, the coefficient of friction of each sheet of media, and a pressure force of the separation belt in a direction normal to the separation pad surface; in practice, the retard/separation force for each sheet of media in the queuing throat of the nip is substantially the same. Accordingly, for sheets of media having a given coefficient of friction, a system designer can design the media separation mechanism to feed plural sheets of media, one at a time through the nip, by selecting a separation belt having a suitable coefficient of friction (relatively large), a frictional surface of the separation pad having a suitable coefficient of friction (relatively large), a suitable contact pressure for the media pick roller/separation belt on the media stack, and a suitable pressure force of the separation belt normal to the separation pad surface.
Although this media separator mechanism (and method) has utility in many applications, it suffers a general drawback of separation pad mechanisms, in that the retard/separation force is directly dependent on the nip force and coefficient of friction of the separator pad. Since both the drive force and separation force are dependent on the nip force, the coefficient of friction of the separator pad is the only independent variable. There are many different types of media having different coefficients of friction, and finding a separator pad material that meets the coefficient of friction requirement along with all of the other physical requirements is difficult.
Media separator mechanisms using media separators having plural friction regions are known. U.S. Pat. No. 5,374,047 discloses a sheet feeder including a media pick and a media separator having a separation pad. The media pick is a single pick roller having a D-shaped friction contact roller. The media separator includes a separation pad holder that holds a separation pad having a high coefficient of friction. The separation pad operates in a manner similar to that described above, to queue plural sheets of media in the nip and feed the plural sheets through the nip one at a time. The separation pad holder also has a projection having a low coefficient of friction located downstream of the separation pad in the feed direction. A support frame supports the separation pad holder and separation pad for movement, against a bias force, in a direction normal to the media pick roller. A sheet of media fed through the nip of the sheet feeder is subsequently nipped/pulled by a downstream pair of feed rollers at a feeding speed higher than a feeding speed through the nip of the sheet feeder; the sheet of media thereby exerts a tension force on the downstream projection of the separation pad in a direction normal to the media pick roller. The separation pad holder and separation pad use the above noted degree of freedom of movement (normal to the media pick roller), to release the sheet of media from a nip force of the sheet feeder.
Although this media separator mechanism (and method) has utility in many applications, it suffers a general drawback of separation pad mechanisms, in that the separation pad uses a single coefficient of friction region for separating sheets of media in the nip.
Media separator mechanisms using a retard roller provide improved reliability of sheet separation for a variety of types of media having different coefficients of friction. U.S. Pat. No. 5,435,538 discloses a retard sheet feeder including a feed roller and a retard roller having a torque limited slip clutch with an integral reversing bias. The retard roller is free to rotate in the feed direction by use of a spring that is axially aligned with the retard roller and allows the retard roller to slip in the feed direction once a predetermined torque level is reached. When the drive torque to the retard roller is reduced, such as when a double sheet is in the drive nip, the torque is not sufficient to overcome the stored spring energy, and the retard roller rotates in a reverse direction by the spring force to drive the double sheet out of the nip. In this manner, plural sheets of media reliably are fed through the retard sheet feeder one at a time.
Although this mechanism and other mechanisms using a retard roller have utility in many applications, such mechanisms suffer a general drawback of retard roller mechanisms, in that such mechanisms require additional elements, such as slip-clutch mechanisms and the like for passive systems and drive motors, controllers, clutch mechanisms and the like for active systems. These additional elements require a significant increase in space, technical maintenance and cost.
A need exists for an improved media separator and media separator mechanism that readily and reliably separate and feed plural sheets of media one at a time. In particular, a need exists for an improved media separator and media separator mechanism that readily and reliably separate and feed different types of media having different coefficients of friction. Further, a need exists for such an improved media separator and media separation mechanism that are compact, simple in design and low cost.